Mitigation of antenna test range impairments caused by presence of undesirable emitters

ABSTRACT

An antenna test range uses direct spread-spectrum based test signals to test the performance of an antenna. Since a spread spectrum waveform has high autocorrelation properties with itself and high cross-correlation properties with signals other than itself, in the receiver coupled to the antenna it is possible to effectively electronically reject all unwanted signals that may be present in the test range, and thereby allow both main beam and off-axis performance of the antenna to be completely and accurately measured.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present invention relates to subject matter disclosed inco-pending U.S. patent application, Ser. No. *, filed coincidentherewith, entitled: “Extension of Dynamic Range of Emitter and DetectorCircuits of Spread Spectrum-Based Antenna Test Range,” by M. Walley etal, assigned to the assignee of the present application, and thedisclosure of which is incorporated herein.

FIELD OF THE INVENTION

[0002] The present invention relates in general to communicationsystems, and is particularly directed to a new and improved antennarange test mechanism that uses direct spread-spectrum based test signalsto mitigate against impairments to an antenna test range caused bymultipath and or the presence of one or more interfering emitters.

BACKGROUND OF THE INVENTION

[0003] The design and testing of a radio wave antenna has historicallybeen principally concerned with the antenna's performance (especiallygain) in the direction of its boresight or main beam axis. For thispurpose, as diagrammatically illustrated in FIG. 1, an antenna 10 whoseperformance is to be measured may be mounted inside a compact test range12, such as an EMI-shielded anechoic chamber, that is configured toeliminate reflections and interference from unwanted sources ofelectromagnetic radiation. Testing the antenna typically involvesdirecting radio wave emissions from a test signal source 14 toward theantenna 10, and measuring the amplitude and phase response of theantenna by means of a range receiver 16, the output of which may bedisplayed or recorded via an associated test and measurement workstation18. As the relative orthogonal principle planes (e.g., azimuth andelevation) parameters between the antenna 10 and test signal source 14are varied (for example, by moving either the antenna or the testsource), both boresight and off-axis gain parameters are derived.

[0004] Unfortunately, at relatively low frequencies, such as UHF, thesize of such a test range becomes physically and cost-wise prohibitive,making it necessary to test the antenna design outdoors. While finding alocation to set up an outdoor antenna test range that is free ofinterferers may not have been particularly difficult several decadesago, it has now become a significant problem due to the proliferation ofwireless commercial products, such as cellular phones and citizen bandradios, as well as specular reflections from buildings and the like.

[0005] Moreover, this interference and reflection free test rangeproblem is compounded by the fact that antenna designers are no longernecessarily principally interested in boresight performance; they nowmust measure the antenna's off-axis characteristics, in order, forexample, to evaluate its ability to place nulls on one or more of thecontinually growing number of interferers, such as the cellular phoneand CB radio devices, referenced above. Thus, the outdoor test rangeoperator could face the dilemma of trying to measure side lobecharacteristics of the antenna, without the presence of one or morelikely interferers, while at the same time designing the antenna toexhibit a characteristic that allows placement of nulls on suchinterferers.

SUMMARY OF THE INVENTION

[0006] In accordance with the present invention the test rangeimpairment problem described above is effectively mitigated by employinga test signal whose characteristics facilitate the signal processing orelectronic rejection of all other signals that may be present in thetest range, and thereby allows both main beam and sidelobecharacteristics of the antenna to be accurately measured. For thispurpose, the present invention uses a test signal, which has very highautocorrelation properties with itself on the one hand for testmeasurement purposes, and high cross-correlation properties with signalsother than itself (especially including interferers and specularreflection) for interference rejection. A signal waveform that readilycomplies with this requirement is a direct sequence spread-spectrumsignal.

[0007] Pursuant to a non-limiting, but preferred embodiment of theinvention, just as in the test ranges of FIGS. 1 and 2, the antennaunder test may be mounted at a location at which measurements are to beconducted by range receiver equipment connected to the antenna. Tomeasure antenna gain and phase parameters for variations in orthogonalprinciple planes, the antenna's response may be measured as a test rangesignal source, that is operative to generate a direct spread-spectrumsignal, is moved relative to the antenna's boresight axis. Conversely,the test source may be fixed and the antenna's pointing angle varied inorthogonal principle planes.

[0008] The test range receiver equipment, to which the output of theantenna under test is coupled, may comprise an RF receiver section whichdemodulates and bandpass filters the spread test signal received fromthe test signal source and outputs a signal that is despread in acorrelation processor to recover the earliest line-of-sight emissionfrom the test source. Multipath is circumvented by selecting theearliest in time (first-to-arrive) correlator output signal, which istime-aligned with the reference PN signal, whose energy content exceedsa prescribed threshold in order to identify the line-of-sight travelingtest signal of interest.

[0009] Impairments due to RF emissions other than those sourced from thespread signal test signal source are avoided, since the energy in thecorrelator outputs for these other emissions is highly cross-correlatedwith the reference PN sequence, and therefore effectively null. Theenergy in the highly autocorrelated output of the correlator processoris digitized and processed by way of the antenna performance measurementalgorithm executed by a test processor.

[0010] Another advantage of the invention is the converse of the above,i.e., the test range signal is highly cross-correlated to licensedtransmitters, and therefore interference of such signals is eliminatedor reduced to a non-interfering level.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 diagrammatically illustrates a compact enclosed antennatest range;

[0012]FIG. 2 diagrammatically illustrates an embodiment of an antennatest range in accordance with the present invention that is configuredto mitigate against the presence of test range impairments;

[0013]FIG. 3 diagrammatically illustrates a direct spread-spectrumsignal based test signal source for use in the antenna test range ofFIG. 2; and

[0014]FIG. 4 diagrammatically illustrates the configuration of rangereceiver equipment for the antenna test range of FIG. 2.

DETAILED DESCRIPTION

[0015] Before describing in detail the new and improved antenna testrange in accordance with the present invention, it should be observedthat the invention resides primarily in a prescribed arrangement ofconventional communication circuits and associated digital signalprocessing components and attendant supervisory control circuitrytherefor, that controls the operations of such circuits and componentsso as to enable both the main beam and off-axis performance of anantenna under test to be accurately measured, irrespective of thepresence of one or more interferers or specular reflectors.

[0016] Consequently, the configuration of such circuits components andthe manner in which they are interfaced with other test range equipmenthave, for the most part, been illustrated in the drawings by readilyunderstandable block diagrams, which show only those specific detailsthat are pertinent to the present invention, so as not to obscure thedisclosure with details which will be readily apparent to those skilledin the art having the benefit of the description herein. Thus, the blockdiagram illustrations are primarily intended to show the majorcomponents of the system in a convenient functional grouping, wherebythe present invention may be more readily understood.

[0017]FIG. 2 diagrammatically illustrates an embodiment of an antennatest range in accordance with the present invention that is configuredto mitigate against the presence of test range impairments, such as butnot limited to specular reflections from a building 31, or signalsemitted from one or more ‘interference’ sources 33, such as a cellularradio tower 34, that may be incident upon an antenna 30 whoseperformance is to be measured.

[0018] As in a typical outdoor test range, the antenna 30 may be mountedat a prescribed location at which measurements are to be conducted byway of associated range receiver equipment 35 connected to the antenna30. Radio wave emissions in the band over which the antenna is operatedare directed from a test signal source 37 toward the antenna 30, and theresponse of the antenna 30 is measured by means of the range receiverequipment 35. To measure the antenna's gain and phase parameters forvariations in orthogonal principle planes, the antenna's response may bemeasured, as the orientation in orthogonal principle planes of theantenna is changed or the test range signal source 37 may be movedrelative to the antenna's boresight axis.

[0019] As described briefly above, the incidence on the antenna 30 ofpotentially impairing emissions or reflections, such reflections 32 froma building 31 and/or emissions from ‘interference’ sources such as acellular radio 33, are mitigated in accordance with the invention byusing a direct spread-spectrum signal as the test signal sourcewaveform. A principal advantage of using a spread-spectrum test signalis the fact that its signature is unique, having high autocorrelationproperties with itself, and high cross-correlation properties withsignals other than itself.

[0020] For this purpose, the test signal source 37 of the test range ofFIG. 2 may be configured as diagrammatically illustrated in FIG. 3,which shows a direct spreading pseudo-random chip sequence generator 40,the output of which is a ‘spread’ or ‘chipped’ data stream having aprescribed number of chips per baud. The chip sequence produced bygenerator 40 is coupled to the test source's RF section 42, which maycomprise an RF mixer to which an RF carrier and the spreading PNsequence output by PN spreading sequence generator 40 are applied, as anon-limiting example. The resulting spread RF test carrier produced bythe RF section 42 is then transmitted via a test source antenna 44 alonga prescribed transmission axis toward the antenna under test.

[0021] A non-limiting example of range receiver equipment, to which theoutput of the antenna under test is coupled, is shown diagrammaticallyin FIG. 4, as comprising an RF receiver-despreader section 50, whichreceives the spread test signal emitted by the test signal source 37 anddespread-correlation processes the received signal to recover theearliest line-of-sight emission from the test source. For this purpose,the receiver section 50 may include a mixer 51 to which the output of alocal oscillator 52 is applied, to provide a baseband spread signal thatis coupled through a bandpass filter 53 to a correlation processor 54.The correlation processor 54 is coupled to receive a spread-spectrumreference signal pattern produced by a pseudo random noise (PN)generator 55. The PN generator 55 is operative to generate the samedirect spreading PN sequence employed by the test signal source of FIG.3, described above.

[0022] Impairments due to multipath are readily avoided by selecting theearliest-in-time correlator output signal whose energy content exceeds aprescribed (adaptive) threshold to identify the first-to-arrive(line-of-sight) test signal of interest. Impairments due to RF emissionsother than those sourced from the test signal source are avoided, sincethe energy in the correlator output for other emissions is highlycross-correlated (rather than highly auto-correlated) with the referencePN sequence, and therefore effectively null. The energy in the highlyautocorrelated (first-to-arrive) output of the correlation processor 54is then digitized and processed by way of the antenna performancemeasurement algorithm executed by a work station 56 associated with therange receiver equipment.

[0023] As will be appreciated from the foregoing description, byemploying a test signal, such as a spread spectrum waveform having highautocorrelation properties with itself and high cross-correlationproperties with signals other than itself to test the characteristics ofan antenna, the test range methodology of the present invention makes itpossible to electronically reject all unwanted signals that may bepresent in the test range, and thereby allows both main beam andoff-axis performance of the antenna to be completely and accuratelymeasured.

[0024] While we have shown and described an embodiment in accordancewith the present invention, it is to be understood that the same is notlimited thereto but is susceptible to numerous changes and modificationsas known to a person skilled in the art, and we therefore do not wish tobe limited to the details shown and described herein but intend to coverall such changes and modifications as are obvious to one of ordinaryskill in the art.

What is claimed:
 1. A method of testing one or more characteristics ofan antenna comprising the steps of: (a) radiating a test signal from atest signal source, so that said test signal is incident upon saidantenna, said test signal having high autocorrelation properties withitself and high cross-correlation properties with signals other thanitself; (b) receiving and demodulating a signal received by saidantenna; (c) correlating a replica of said test signal with the signalreceived and demodulated in step (b), so as to extract energy in saidtest signal and exclude energy in any other signals that may be incidentupon said antenna; and (c) processing test signal energy extracted instep (b) to derive a measure of said one or more characteristics of saidantenna.
 2. A method according to claim 1 , wherein said test signalcomprises a spread spectrum signal.
 3. A method according to claim 2 ,wherein step (a) comprises radiating said test signal from said testsignal source at a plurality of spaced apart signal source locationshaving respectively different orthgonal principle planes parametersrelative to the boresight of said antenna.
 4. An antenna test rangecomprising: a test signal source, spaced apart from an antenna undertest and being operative to emit a test signal that is incident uponsaid antenna under test, said test signal having high autocorrelationproperties with itself and high cross-correlation properties withsignals other than itself; a receiver coupled to said antenna undertest, and being operative to demodulate a signal received by saidantenna under test, and to correlate a replica of said test signal withthe demodulated signal so as to extract energy in said test signal andexclude energy in any other signals that may be incident upon saidantenna under test; and a signal processor, coupled to said receiver,operative to process the test signal energy extracted by said receiverand derive a measure of one or more characteristics of said antennaunder test.
 5. An antenna test range according to claim 4 , wherein saidtest signal comprises a spread spectrum signal.